When drilling into a subterranean formation in connection with the extraction of hydrocarbons, it is important to know the pore pressure (also known as formation pressure). In order to avoid water, hydrocarbon gas or oil flowing out of a well while it is being drilled, offsetting pressure in the hole is controlled by the use of drilling mud. The pressure produced by the mud is designed to be greater than the estimated pore pressure of the formations being drilled. The magnitude of the pressure exerted by the mud is controlled by varying the density of the mud by adding appropriate amounts of weighting material such as barite or hematite. In addition, the pressure exerted by pumps which pump the drilling mud can be varied. More recent techniques such as managed pressure drilling exert a back-pressure on the mud system. The pressure maintained on the mud system must equal or exceed the pore or formation pressure in order to prevent an untimely exit of fluids which is a potentially dangerous event in addition to being wasteful of natural resource. The pore pressure tends to vary along the length of the well, which adds to the complexity.
Current techniques for pore pressure estimation during drilling in sedimentary shale formations include the use of calculations based on measurements of rock resistivity, sonic travel time or velocity, and density (Eaton, 1975; Mouchet and Mitchell, 1989; Bowers, 1994). These measurements are obtained using logging tools run in the hole either while drilling, known as logging while drilling (LWD), or following drilling, which is commonly referred to as wireline logging. These pore pressure calculations assume that the pores within shale or clay rocks are filled with water and have very little free oil or gas in the pores, and are comprised of less than a few percent organic matter incorporated in the original sediment.
LWD and wireline logging produce a continuous plot or curve of data over the length of the well, or over that part for which data has been gathered. From any one of these curves, it has been possible to derive reasonably accurate values of pore pressure. These calculations can be verified or at least supported by direct pressure measurements obtained from more permeable rocks such as sandstone and limestones interstratified with the shale obtained using tools such as wireline formation testers either following drilling or while drilling (Gunter and Moore, 1987; Hooper et. al., 1999; Finneran et. al., 2005). These measurements cannot be taken continuously due to expense and they only work in relatively porous and permeable rocks interstratified with rocks lacking sufficient permeability to liquid or gas flow. These tools cannot directly take pressure measurements in shale and clay rocks due to their very low permeability relative to most sandstone and limestone rocks. Therefore calculations to estimate pore pressure based on log response in shale and clay rocks are more commonly used to monitor pore pressure rather than direct pressure measurements.
Besides direct pressure measurements qualitative estimates of pore pressure while drilling can be based on the relative volume of natural gas in the mud system and observations of the volume and character of cuttings, the small rock chips removed by the drill bit, and other rock samples brought up from the bottom of the well. In general small volumes of natural gas are present in most rocks in the subsurface perhaps mostly dissolved in water in the pore space. It is expected that small volumes of natural gas will be released simply when the drill bit excavates rock as the well is drilled into the earth. When larger than expected volumes of natural gas are detected this indicates that some additional source of gas is active which is flowing or seeping into the well. The conclusion is that at some point in the well the pressure exerted by the drilling mud is less than the formation pressure allowing excess fluid and natural gas to seep or flow into the well. This provides a means of estimating the pore pressure and, although qualitative, it provides an additional means to confirm or support the well log based calculations of pore pressure (Mercer, 1963). In addition to the volume of gas, cuttings that appear to be larger than expected for the type of bit in the hole or which have a morphology indicative of rock failure can suggest that the formation pore pressure is less than the pressure exerted by the mud (Mouchet and Mitchell, 1989).
A problem arises in shale formations rich in organic matter or free oil or gas where it is not possible to take direct measurements of pressure due to the inherent low permeability and where qualitative approaches such as gas volumes and cuttings evaluation can be insufficient. Organic matter, or free oil or gas in shale rock, usually referred to as source rocks or unconventional gas shales, will affect the log responses used for pore pressure calculations independent of the pore pressure or porosity state of the shale rock therefore detrimentally affecting the accuracy of the pore pressure calculation. The effect of organic matter and free oil and gas in shale rock has been known for a number of years (Passey, et. al., 1990). In general organic matter and free oil and gas will greatly increase the resistivity measurement of the shale relative to a water-wet shale under the same down-hole conditions, as well as increase the travel time, reduce the velocity, and decrease the density. This affect will lead to an under-estimation of the pore pressure based on the resistivity measurements in an organic or shale gas rocks using the accepted equations, and will result in an over-estimation of the pore pressure based on the sonic or density log measurements
There is therefore a need for a relatively inexpensive technique for assessing pore pressure in shale formations, especially those types of shale formation mentioned above. The technique would ideally be applicable not only to shale formations of the type which tend to produce inaccurate results from log data, but also to any other shale formation, so that the same technique may be used without knowing in advance what type of shale formation is being analysed.